This invention relates to a surface acoustic wave device for transmitting surface acoustic waves along a surface between a first transducer and a second transducer. In such devices the transducers are normally formed by a series of interdigitated electrodes which are produced by a photolithographic process using a master exposure mask. The exposure mask needs to be made by sophisticated pattern generating equipment since the widths of the interdigitated electrodes are commonly in the range of 2 .mu.m to 0.5 .mu.m. The cost of making such masks is very high.
In order to make different surface acoustic wave devices which operate at respective different frequencies it has in the past been necessary to use different masks. Accordingly, when a need has arisen for a surface acoustic wave device operating at a particular frequency it has hitherto been necessary to include the cost of manufacturing an appropriate mask in the cost of the product. This invention arose with the aformentioned problem in mind.
The invention provides a surface acoustic wave device comprising: means for transmitting surface acoustic waves along a surface between a first transducer whose efficiency has a plurality of peaks at a first set of different frequency values and a second transducer whose efficiency has a plurality of peaks at a second set of different frequency values; means influencing the velocity of the waves in at least a region of the first transducer so as to displace the first set of frequency values relative to the second set thereby making one of the frequency values of the first set substantially coincide with one of the frequency values of the second set so that the device has a good frequency response at that frequency value, and other frequency values of the first set not coincide with frequency values of the second set so that the device does not have a good frequency response at the other values.
The velocity can be controlled in one or more of a number of different ways to be described later.
Each transducer is preferably formed by a plurality of groups of interdigitated electrodes, the groups being separated by a distance greater than the distance between electrodes of a given group. This type of electrode structure is able to provide the plurality of peak frequency values required. The surface acoustic wave velocity can be adjusted only to a limited extent, normally less than 3% and, accordingly, each transducer is preferably constructed so that any one of said frequency values differs from an adjacent frequency value by not more than 4 and preferably not more than 3% thereof.
In a preferred form of the invention, each pair of adjacent frequency values of the first set are separated by the same first frequency spacing and each pair of adjacent frequency values of the second set are separated by the same second frequency spacing which is different from the first frequency spacing. If the difference in frequency spacing is sufficiently small, then an appropriate velocity adjustment in the region of one of the transducers will cause the frequency values of the appropriate set to "slide" in relation to those of the other set in a manner similar to that of a vernier until coincidence of a particular frequency value of one set occurs with a frequency value of the other set.
The adjustment of velocity in a region of the first or second transducer will result in a finished product which may have different surface acoustic wave properties in the regions of the different transducers. The means for influencing the velocity may include a metal layer between electrodes of a transducer, the area or thickness or other dimension of this metal layer influencing the velocity. If the appropriate transducer contains a number of groups of interdigitated electrodes, this metal layer is preferably between the groups. Another possibility for influencing the velocity is to apply a layer of insulating material over one or both transducers. Another preferred possibility is to alter or control the thickness of the electrodes of the transducers.
While it would be possible to manufacture transducers which, in the first place, had characteristics giving the required velocity, it is more convenient to carry out a step of velocity adjustment. This can be done, in the case of the aforementioned metal layer or electrodes, by gradually eroding the metal layer while observing the frequency characteristics of the device. In the case of the use of a layer of insulating material then this can be gradually applied or removed, again while observing frequency characteristics.